Ectothermy and Endothermy
Ectothermy and
Endothermy
The terms “cold-blooded” and “warmblooded” have long been used to divide animals into two groups: invertebrates and vertebrates that feel cold to the touch, and those, such as humans, other mammals, and birds, that do not. It is true that body temperature of mammals and birds is usually (though not always) warmer than the air temperature, but a “cold-blooded” animal is not necessarily cold. Tropical fishes, and insects and reptiles basking in the sun, may have body temperatures equaling or surpassing those of mammals. Conversely, many “warmblooded” mammals hibernate, allowing their body temperature to approach the freezing point of water. Thus the terms “warm-blooded” and “cold-blooded” are hopelessly subjective and nonspecific but are so firmly entrenched in our vocabulary that most biologists find it easier to accept the usage than to try to change people.
The term poikilothermic (variable body temperature) and homeothermic (constant body temperature) are frequently used by zoologists as alternatives to “cold-blooded” and “warm-blooded,” respectively. These terms, which refer to variability of body temperature, are more precise and more informative, but still offer difficulties. For example, deep-sea fishes live in an environment having no perceptible temperature change. Even though their body temperature is absolutely stable, day in and day out, to call such fishes homeotherms would distort the intended application of the term. Furthermore, among the homeothermic birds and mammals there are many that allow their body temperature to change between day and night, or, as with hibernators, between seasons.
Physiologists prefer yet another way to describe body temperatures, one that reflects the fact that an animal’s body temperature is a balance between heat gain and heat loss. All animals produce heat from cellular metabolism, but in most the heat is conducted away as fast as it is produced. In these animals, the ectotherms—and the overwhelming majority of animals belong to this group—body temperature is determined solely by the environment. Many ectotherms exploit their environment behaviorally to select areas of more favorable temperature (such as basking in the sun) but the source of energy used to increase body temperature comes from the environment, not from within the body. Alternatively some animals are able to generate and retain enough heat to elevate their own body temperature to a high but stable level. Because the source of their body heat is internal, they are called endotherms. These favored few in the animal kingdom are the birds and mammals, as well as a few reptiles and fast-swimming fishes, and certain insects that are at least partially endothermic. Endothermy allows birds and mammals to stabilize their internal temperature so that biochemical processes and nervous system functions can proceed at steady high levels of activity. Endotherms can thus remain active in winter and exploit habitats denied to ectotherms.
The terms “cold-blooded” and “warmblooded” have long been used to divide animals into two groups: invertebrates and vertebrates that feel cold to the touch, and those, such as humans, other mammals, and birds, that do not. It is true that body temperature of mammals and birds is usually (though not always) warmer than the air temperature, but a “cold-blooded” animal is not necessarily cold. Tropical fishes, and insects and reptiles basking in the sun, may have body temperatures equaling or surpassing those of mammals. Conversely, many “warmblooded” mammals hibernate, allowing their body temperature to approach the freezing point of water. Thus the terms “warm-blooded” and “cold-blooded” are hopelessly subjective and nonspecific but are so firmly entrenched in our vocabulary that most biologists find it easier to accept the usage than to try to change people.
The term poikilothermic (variable body temperature) and homeothermic (constant body temperature) are frequently used by zoologists as alternatives to “cold-blooded” and “warm-blooded,” respectively. These terms, which refer to variability of body temperature, are more precise and more informative, but still offer difficulties. For example, deep-sea fishes live in an environment having no perceptible temperature change. Even though their body temperature is absolutely stable, day in and day out, to call such fishes homeotherms would distort the intended application of the term. Furthermore, among the homeothermic birds and mammals there are many that allow their body temperature to change between day and night, or, as with hibernators, between seasons.
Physiologists prefer yet another way to describe body temperatures, one that reflects the fact that an animal’s body temperature is a balance between heat gain and heat loss. All animals produce heat from cellular metabolism, but in most the heat is conducted away as fast as it is produced. In these animals, the ectotherms—and the overwhelming majority of animals belong to this group—body temperature is determined solely by the environment. Many ectotherms exploit their environment behaviorally to select areas of more favorable temperature (such as basking in the sun) but the source of energy used to increase body temperature comes from the environment, not from within the body. Alternatively some animals are able to generate and retain enough heat to elevate their own body temperature to a high but stable level. Because the source of their body heat is internal, they are called endotherms. These favored few in the animal kingdom are the birds and mammals, as well as a few reptiles and fast-swimming fishes, and certain insects that are at least partially endothermic. Endothermy allows birds and mammals to stabilize their internal temperature so that biochemical processes and nervous system functions can proceed at steady high levels of activity. Endotherms can thus remain active in winter and exploit habitats denied to ectotherms.
Support our developers
